you can do a big water change. I do 50% 1x or 2x a week and I can do closer to 75 0r 100% with out a problem. BUT you need to age your water for at least 8 hours, i age for 24 hours, get the temp correct, and of course dechlorinate. if you don't have any buffering going on in your tank the pH shouldn't be a problem.
-try cleaning your filter in old tank water, or new water that has been dechlorinated, maybe it is just dirty and full of decomposing junk